TM 55-4920-401-13&PFigure 1-23. Insulation Check Circuit (Simplified Schematic)board.(9) Resistance Check Circuit. This circuit(fig. 1-24), located in the probe controller, is used tocheck and adjust the resistance of the aircraft ther-mocouple circuit. It consists of a Wheatstone bridgewith the RESISTANCE CHECK meter serving asthe galvanometers. One leg of the bridge contains aprecision wirewound resistor that is selected withthe RESISTANCE % A/C INDICATOR CHECKswitch. The aircraft thermocouple circuit (less theindicator) is placed in the other leg of the bridge.The aircraft resistance spool is then adjusted untilthe galvanometer is balanced.(10) Power Supplies and Power Distribu-tion. Primary line voltage (115 vat. 50-400 Hz) isapplied to the POWER INPUT receptacle on theprobe controller. The voltage is applied through theinterconnect cable and the MASTER POWERswitch to the primary windings of T1 in the trim-mer. There are five secondary windings on trans-former T1 (fig. 1-25). All secondary windings arecenter-tapped. Only switch positions that switchpower are shown in figure 1-25.(a) Twenty-Eight VDC Supply. This sup-ply consists of the circuitry of CR9, CR10, and Q10-Q12 on the switch-power board. It is referenced tothe -15 vdc supply. The 28 vdc supply excites theSTD DAY lamp and its control circuit. It is alsoswitched by the FUNCTION SELECT switch to the1-23